The method of the instant invention provides for preparation of porous PAEK materials modified with target functional groups including polar groups, such as hydroxyl groups, —OH, or primary, secondary or tertiary amino groups, —NH2, ═NH, ≡NR, negatively or positively charged ionic groups, such as —SO3−, —COO−, and —NH4+ groups, hydrophobic groups such as siloxane or perfluorcarbone groups, and non-polar groups, such as linear or branched hydrocarbon groups. Porous PAEK articles can be modified with target functional groups throughout the porous body of the article or the modification can be limited to the surface of the porous article only. In one embodiment of this invention the porous PAEK article is formed first followed by the surface functionalization. In another embodiment of this invention, a porous PAEK article is formed by functionalizing the surface of a non-porous PAEK article first followed by the formation of the internal porous structure. The functionalized porous PAEK article can be in the form of a flat sheet, a rod, a sphere, or a tube. Functionalized PAEK materials of complex shapes can be further prepared following the teaching of the present invention, including a hollow fiber configuration. Functionalized porous PAEK articles of this invention can be used as a porous media for a broad range of applications, including porous membranes for fluid separations, such as microfiltration, nanofiltration, ultrafiltration and gas separation, as membrane bioreactors and membrane contactors, as battery separators and as a sorption media.
Porous polymeric membranes are well known in the art and are used widely for filtration and purification processes, such as filtration of waste water, preparation of ultra pure water and in medical, pharmaceutical or food applications, including removal of microorganisms, dialyses and protein filtration. Porous polymeric membranes are also used as contactors to facilitate dissolution of gases in liquids or to remove gases from liquids, as membrane bioreactors, and in numerous other applications where they serve as a generic phase separator, for example, as a battery separator. While these membranes have found broad utility for a variety of purposes, they suffer from several disadvantages: broad and non uniform pore size distribution, limited chemical, solvent and thermal resistance, and surface characteristics that do not meet target application requirements. For example, porous ultrafiltration membranes are frequently subject to fouling due to the dissolved solute adsorption on the membrane surface. Thus membranes with hydrophilic anti-fouling surface characteristics are still required. Porous polyolefin membranes, such as polypropylene and polyethylene membranes, are utilized as membrane contactors to promote dissolution or removal of gases from liquids. However, these membranes frequently wet out by the liquid media which leads to reduction in mass transfer and an inferior performance. Porous membranes with improved surface properties are thus required for continuous stable operation of membrane contactors. Furthermore, commercial porous membranes exhibit limited solvent resistance that limits the scope of their application. Porous membranes with tailored surface characteristics, uniform pore size distribution, improved thermal stability and solvent resistance are thus still needed.
Poly(aryl ether ketone)s represent a class of semi-crystalline engineering thermoplastics with outstanding thermal properties and chemical resistance. One of the representative polymers in this class is poly(ether ether ketone), PEEK, which has a reported continuous service temperature of approximately 250° C. PAEK polymers are virtually insoluble in all common solvents at room temperature. These properties make PAEK attractive materials for porous membrane preparation. However, application of PAEK polymers to fabrication of membranes has been limited owing to their intractability, which prevents the use of conventional solvent-based methods of membrane casting.
PAEK polymers can be chemically modified to impart functionality, for example, by sulfonation. However, articles formed from such functionalized PAEK polymers lose many of the desired properties. Bulk modification leads to a disruption in polymer chain crystallization and articles subsequently formed from such functionalized polymers loose solvent resistant properties. Chemical resistance of PEAK polymers makes the functional modification of the preformed porous article difficult and such functionalized porous PAEK articles are virtually unknown.
A number of methods to prepare porous PAEK membranes have been disclosed in the art. It is known to prepare porous PEEK membranes from solutions of strong acids, such as concentrated sulfuric acid. However, PEEK can undergo sulfonation in the concentrated sulfuric acid media and thus can loose some of its desirable sought after properties. U.S. Pat. No. 6,017,455 discloses preparation of non-sulfonated porous PEEK membranes from concentrated sulfuric acid solvents sufficiently diluted by water to prevent sulfonation. The membranes are formed by casting PEEK solution to form a film followed by coagulation in a concentrated sulfuric acid. This membrane preparation process is complicated and produces large amounts of waste acid.
U.S. Pat. No. 5,997,741 discloses preparation of porous PEEK membranes by forming a solution of PEEK polymer in a concentrated sulfuric acid at the temperature of 15° C. or lower to prevent sulfonation. The solution is processed and cast at a sub ambient temperature, followed by coagulation in water or in a concentrated sulfuric acid. Only dilute PEEK solutions can be formed in the concentrated sulfuric acid which adversely affects film forming characteristics, the mechanical characteristics, and the pore morphology of the porous PEEK membranes.
U.S. Pat. Nos. 4,992,485 and 5,089,192 disclose preparation of PEEK membranes from non-sulfonating acid solvents that include methane sulfonic acid and trifluoromethane sulfonic acid. European Patent Specification EP 0 737 506 discloses preparation of improved polymeric membranes based on PEEK admixtures with polyethylene terephthalate. The membranes are formed by the solution casting process from a methane sulfuric acid/sulfuric acid solvent mixture.
The acid based solvent systems for manufacturing of porous PEEK membranes disclosed in the art are highly corrosive, frequently toxic and generate substantial environmental and disposal problems. For these and other reasons, the acid based casting processes have found limited commercial use.
An alternative to the acid based solvent system for PEEK membrane preparation involves the use of high boiling point solvents and plasticizers that dissolve PEEK polymer at elevated temperatures. U.S. Pat. Nos. 4,957,817 and 5,064,580, both issued to Dow Chemical Co., disclose preparation of porous PEEK articles from its admixture with organic polar solvents having a boiling point in the range of 191° C. to 380° C., such as benzophenone and 1-chloronaphthalene, and organic plasticizers capable of dissolving at least 10 weight percent of PEEK, respectively. The final porous article is formed by removing the organic polar solvents and/or plasticizers by dissolution into a low boiling temperature solvent.
U.S. Pat. No. 5,200,078 discloses preparation of microporous PEEK membranes from its mixtures with plasticizers wherein the membrane undergoes a drawing step prior to or after the plasticizer is removed by leaching.
U.S. Pat. No. 5,227,101 issued to Dow Chemical Co. discloses preparation of microporous membranes from poly(aryl ether ketone) type polymer by forming a mixture of PEEK type polymer, a low melting point crystallizable polymer, and a plasticizer, heating the resulting mixture, extruding or casting the mixture into a membrane, quenching or coagulating the membrane and leaching the pore forming components.
U.S. Pat. No. 5,205,968, issued to Dow Chemical Co., discloses preparation of microporous membranes from a blend containing a poly(aryl ether ketone) type polymer, an amorphous polymer and a solvent.
M. F. Sonnenschein, in the article entitled “Hollow fiber microfiltration membranes from poly(ether ether ketone)”, published in the Journal of Applied Polymer Science, Volume 72, pages 175-181, 1999, describes preparation of PEEK hollow fiber membranes by thermal phase inversion process. The use of a leachable additive polymer, such as polysulfone, is proposed to enhance membrane performance. Preparation of porous PEEK membranes by coextrusion of PEEK with polysulfone polymers followed by the dissolution of the polysulfone polymer from the interpenetrating network is disclosed in European Patent Application 409416 A2.
It is also known in the art to prepare porous PEEK membranes from its blends with a compatible poly(ether imide) polymer, PEI. Preparation of such membranes is described by R. S. Dubrow and M. F. Froix in U.S. Pat. No. 4,721,732 and by R. H. Mehta et al. in an article entitled “Microporous membranes based on poly(ether ether ketone) via thermally induced phase separation”, published in the Journal of Membrane Science, Volume 107, pages 93-106, 1995. The porous structure of these PEEK membranes is formed by leaching the poly(ether imide) component with an appropriate strong solvent such as dimethylformamide. However, as described by Mehta et al., the quantitative removal of PEI component by leaching is essentially impossible which in turn can lead to an inferior membrane performance.
Japan Kokai Tokkyo Koho 91273038 assigned to Sumitomo Electric Industries, Ltd., discloses preparation of porous PEEK membranes by an ion track etching method.
M. L. Bailey et al. in U.S. Pat. No. 5,651,931 describe a sintering process for the preparation of biocompatible filters, including PEEK filters. The filters are formed from a PEEK powder of a pre-selected average particle size by first pressing the powder into a “cake” followed by sintering in an oven or furnace. The process is limited to preparation of filters with a relatively large pore size and a broad pore size distribution and does not provide economic means of forming large membrane area fluid separation devices.
A number of techniques have been used in the art to chemically modify the surface of dense PEEK films to affect surface characteristics such as friction, wettability, adsorption and adhesion, including cell adhesion. O. Noiset, et al., have modified the PEEK film surface using wet-chemistry technique by selectively reducing ketone groups to form hydroxyl groups and then covalently fixing hexamethylene diisocyanate by addition onto the hydroxyl function (Journal of Polymer Science, Part A, Vol. 35, pages 3779-3790, 1997). C. Henneuse-Boxus, et al., have modified PEEK film surfaces using photochemical routes (European polymer Journal, Vol. 37, pages 9-18, 2001). P. Laurens, et al., have modified PEEK surfaces with excimer laser radiation (Applied Surface Science, Vol. 138-139, pages 93-96, 1999). N. Frauchina and T. McCarthy have modified semi-crystalline PEEK films with carbonyl-selective reagents to induce surface functionality (Macromolecules, Vol. 24, pages 3045-3049, 1991). The surface modified films were robust and unaffected by a variety of solvents.
In U.S. Pat. No. 5,260,415, I. David disclosed a process for the crosslinking of polymer containing diaryl ketone groups by heating the polymer with alcohol and/or alkoxide to enhance chemical resistance.
However, there is no disclosure in the prior art of a simple and commercially scalable process for the preparation of functionalized porous PAEK articles including porous PAEK membranes and use thereof.